This invention relates to an epitaxial growth substrate comprising a base material composed of a sapphire substrate, a SiC substrate, a GaN substrate or the like and an AlxGayInzN film (x+y+z=1,x greater than 0,y,zxe2x89xa70) on the base material, and a method for producing the same.
An epitaxial growth substrate is employed for an electronic device such as a light emitting diode (LED), a laser diode (LD) or a field effect transistor (FET). In producing such an electronic device, semiconductor films such as AlxGayInzN film (x+y+z=1,x,y,zxe2x89xa70) are formed on the epitaxial growth substrate by epitaxial growth. Attention is paid to an AlxGayInzN film (x+y+z=1,x,y,zxe2x89xa70) because it has a large bandgap to generate and emit a short wavelength light in a light emitting element.
FIG. 1 is a cross sectional view showing a conventional light emitting diode to generate a blue light, which is composed of the above AlxGayInzN films.
For example, a GaN film 2 as a buffer layer is formed on a C-face sapphire (Al2O3) substrate 1 at a low temperature by CVD, and an n-type AlxGayInzN film 3 is formed on the GaN film 2 by epitaxial growth through CVD. Then, a p-type AlxGayInzN film 4 is epitaxially grown on the n-type AlxGayInzN film 3 by epitaxial growth through CVD, and a low resistive p-type AlxGayInzN film 5 is epitaxially grown on the p-type AlxGayInzN film 4.
Electrodes 6 and 7 are formed on the n-type AlxGayInzN film 3 and the p-type AlxGayInzN film 5, respectively.
If the n-type AlxGayInzN film 3 is directly formed on the sapphire substrate 1 by CVD, it has a large amount of defects, degraded crystallinity and a relatively rough surface. Therefore, in this case, the light emitting diode having the directly formed n-type AlxGayInzN film can have a relatively small light emission efficiency.
Therefore. as shown in FIG. 1, the GaN film 2, which is formed at a low temperature by CVD, intervenes as th e buffer layer between the sapphire substrate 1 and the n-type AlxGayInzN film 3. Since the GaN film 2 is grown at a low temperature by CVD, the lattice constant difference of approximately 10% between the sapphire substrate 1 and the n-type GaN film 3 is compensated. Moreover, it is proposed that an AlN film is employed as the buffer layer instead of the GaN film 2.
Moreover, instead of forming the GaN film or the AlN film by epitaxial growth at a low temperature by CVD, the sapphire substrate 1 is exposed to an ammonia atmosphere under a high temperature of 1000xc2x0 C. to nitride its surface and form an AlN film with a thickness of 100 xc3x85 or below thereon. The AlN film serves as the above buffer layer, so that the lattice constant difference between the sapphire substrate 1 and the GaN film 3 is compensated.
The present invention is directed at providing an epitaxial growth substrate comprising a base material composed of a sapphire substrate, a SiC substrate, a GaN substrate or the like and a AlxGayInzN film (x+y+z=1,x,y,zxe2x89xa70) formed on one main surface of the base material. However, as mentioned above, if the AlxGayInzN film is directly formed on such a sapphire substrate by CVD, it can have only a degraded crystallinity and a relatively rough surface.
Then, using the above epitaxial growth technique for the above GaN film, a GaN film or an AlN film is formed, as a buffer layer, in a thickness of 1001-300 xc3x85 on the sapphire substrate by epitaxial growth at a low temperature by CVD. As a result, the AlxGayInzN film, formed on the buffer layer, can have a relatively good crystallinity and a flat surface. However, the crystallinity of the AlxGayInzN film is sensitive to the buffer layer configuration such as thickness and the buffer layer-forming conditions, and the buffer layer itself is formed at a low temperature, so that the crystallinity of the AlxGayInzN film can not be sufficiently improved because of its low crystallization.
Moreover, when an n-type AlxGayInzN (x+y+z=1,x,y,zxe2x89xa70) film is formed, by epitaxial growth at a high temperature by CVD, on an AlN film having a thickness of 100 xc3x85 or below formed by the exposure of the sapphire substrate to a high temperature ammonia atmosphere, it can have excellent crystallinity, but usually does not have a flat surface. The rough surface flatness may result from the thermal treatment at a high temperature in forming the AlN film. On the other hand, the high crystallinity may result from the crystallinity of the sapphire substrate because the AlN film is obtained by improving the surface layer of the substrate.
The above rough surface flatness and high crystallinity is observed for an AlxGayInzN (x+y+z=1,x,y,zxe2x89xa70) film formed on a SiC substrate and a GaN substrate instead of the sapphire substrate. In this way, the above-mentioned conventional technique can provide only the AlxGayInzN film having excellent surface flatness but degraded crystallinity, or the AlxGayInzN film having excellent crystallinity through a small amount of defects but degraded surface flatness. Up to now, the AlxGayInzN film having both excellent crystallinity and surface flatness could not be provided.
It is an object of the present invention to provide an epitaxial growth substrate on which the AlxGayInzN (x+y+z=1,x,y,zxe2x89xa70) film having both excellent crystallinity through a small amount of defects and surface flatness formed by epitaxial growth.
It is another object of the present invention to provide a method for producing the above epitaxial growth substrate.
This invention relates to an epitaxial growth substrate comprising a base material and an AlxGayInzN (x+y+z=1,x,y,zxe2x89xa70) film, on one surface of the base material, of which the opposite surface to the base material is polished and thus, flattened.
Since the AlxGayInzN film constituting the epitaxial growth substrate of the present invention is epitaxially grown and flattened by polishing, it can have its excellent crystallinity and surface flatness. Accordingly, when an additional AlxGayInzN (x+y+z=1,x,y,zxe2x89xa70) film is formed on the epitaxial growth substrate by epitaxially growth, it can have excellent crystallinity and surface flatness.
For enhancing the crystallinity and surface flatness of the additional AlxGayInzN film, it is preferable that the epitaxial growth substrate has a high crystallinity nitride film between the base material and the AlxGayInzN film.
This invention also relates to a method for producing an epitaxial growth substrate comprising the steps of:
preparing a base material,
epitaxially growing an AlxGayInzN (x+y+z=1,x,y,zxe2x89xa70) film on one surface of the base material, and
polishing and thus, flattening the opposite surface of the AlxGayInzN film on the base material.
According to the producing method of the present invention, the above epitaxial growth substrate can be easily and precisely formed. Therefore, in this case, it is preferable to form a high crystallinity nitride film between the base material and the AlxGayInzN film, in view of the above enhancement for the crystallinity and surface flatness of the additional AlxGayInzN film.